A known vehicular navigation device (e.g., refer to Japanese Patent 2778374, JP-A-2006-170970, JP-A-2007-271602, JP-A-2007-47271, and JP-A-6-27876) has a road learning function such that when a vehicle travels on a road unregistered in map data, the vehicular navigation device detects the shape of the unregistered road and registers the detected shape in the map data. Such a vehicular navigation device has a technical thought to obtain a travel locus using various sensors such as a GPS satellite system and detect the shape of a road based on the obtained travel locus. The vehicular navigation device is further configured to store the travel locus between a start point and an end point automatically and register an unregistered new road automatically.
More specifically, Japanese Patent 2778374 discloses a navigation device configured to detect a travel distance of a vehicle, detect a traveling direction of the vehicle, and calculate a travel locus of the vehicle based on the detected travel distance and the detected traveling direction at a predetermined travel distance interval. The navigation device is further configured to calculate an estimated position of the vehicle based on road map data and the calculated travel locus of the vehicle and indicate the estimated position of the vehicle on a road map around the vehicle. The navigation device has a function such that when the vehicle travels on an unregistered road, which is not registered in a map storage device, the navigation device converts data format of a travel locus of the unregistered road calculated by a travel-locus calculation unit into data format of road map data stored in the map storage device. In this way, the navigation device stores the travel-locus data of the unregistered road.
JP-A-2006-170970 discloses a method to determine a point through which a self-vehicle departs from a registered road stored in map data. The JP-A-2006-170970 discloses a method such that when the vehicle travels on a road toward a T-junction and when the position of the vehicle does not coincide with a road position, the navigation device performs a map matching operation to calculate a map matching position, which coincides with the road position. When the vehicle starts traveling on a road, which is not stored in the map data, the navigation device calculates the map matching position to coincide with a road near the self-vehicle.
JP-A-2007-271602 discloses a technical thought to shift a travel locus in parallel, while a self-vehicle is traveling on an unregistered road, such that a start point of the travel locus including the unregistered road coincides with the coordinates where detection of the unregistered road is started. Thereafter, the travel locus is affine-transformed such that the end point of the travel locus coincides with the coordinates of a detection end point of the unregistered road. Thereby, the travel locus including the unregistered road is obtained. In this technical thought, when a road verification unit and the travel locus show a matching state, it is determined whether the travel locus is within the road width. When it is determined that the travel locus is not within the road width, a road departure point is set to the coordinates of the present position of the vehicle. In this case, the road departure point is set on a registered road rearward by a predetermined distance.
JP-A-2007-47271 discloses a technical thought to connect the self-positions so as to obtain a travel locus as a new road, when the map matching of the self-position cannot be carried out, and update road information. Thereafter, the new road is evaluated according to a deviation between the self-position and the new road. According to this technical thought, congruent points between both data are obtained in consideration of the difference between the length of a polygonal line of the update information and the length of a polygonal line obtained from a position detection result in the evaluation. Further, coordinates average of the congruent points are obtained as a correction result. In this way, an intermediate position between update information and the position detection result in the evaluation can be calculated as the correction result.
It is noted that, in general, a vehicle travels on one side of a road. That is, when a vehicle travels on a road, a road shape deviated from a center position of an intersection may be detected. When a vehicle turns on an unregistered road such as an intersection, a travel locus of the vehicle may be in a curved shape. Consequently, a road shape different from an actual road shape may be stored. Such a difference may be significant in a right-angled intersection, for example.